Field of the Invention
The present invention relates generally to the construction and configuration of a stator for electrical machines including both motors and generators, and more particularly to an improved stator construction utilizing a stator core in the form of a cylindrical ring lacking inwardly projecting teeth made of a magnetizable material, with the windings of the stator being supported by a support structure made of non-magnetizable material effectively increasing the diameter of the magnetic air gap, the present invention providing a suitable stator structure for use with high magnetic energy permanent magnet rotors.
In the design and construction of permanent magnet machines, there are two considerations which dictate the design of an improved permanent magnet machine having great appeal to potential purchasers. These two factors are, first, the desire to minimize the cost of the machine, or to provide the most machine for the least money, and secondly to increase machine efficiency while reducing machine size by utilizing smaller sized higher energy product permanent magnets.
With regard to the first of these factors, the desire to minimize construction cost of a permanent magnet machine, it is apparent that the most labor-intensive component of a permanent magnet machine to assemble is the stator, which is typically assembled by winding coils of wire around teeth contained on an iron stator core. In cross-section, a typical stator core resembles a cylinder with T-shaped teeth extending radially inwardly. It may be appreciated that winding the stator windings onto the T-shaped teeth of the stator core is therefore a labor intensive, and hence expensive, process.
It is apparent that in order to achieve a significant reduction in the cost of assembling a permanent magnet machine, it is necessary to redesign the stator to reduce the amount of labor needed to wind the stator windings onto the stator core. Accordingly, this accomplishment is an object of the present invention.
In recent years, high energy product permanent magnets representing significant energy increases over previously known permanent magnets have become available. For example, samarium cobalt permanent magnets having an energy product of 27 megagauss-oersted (MGO) have recently become available. In addition, neodymium-iron-boron magnets have recently become available which have an energy product of 35 MGO, and it appears that in the near future an energy product of at least 45 MGO will be achieveable by advanced permanent magnets.
A rotor making the maximum use of high energy product permanent magnets is disclosed in Assignee's concurrently filed U.S. patent application Ser. No. 800,183, issued May 19, 1987 as U.S. Pat. No. 4,667,123 entitled "Two Pole Permanent Magnet Rotor Construction for Toothless Stator Electrical Machines", which patent application is hereby incorporated herein by reference.
Theoretically, the use of such high energy product permanent magnets should permit increasingly smaller machines to be built which will be capable of supplying increasingly high power outputs. However, for a particular power output a smaller machine would have approximately the same amount of losses as a larger machine, and since its size is smaller the losses per volume of machine would be higher, resulting in a high watts loss density.
Such a high watts loss density would make direct liquid cooling virtually mandatory in order to allow the smaller machine to operate on a continuous basis. Unfortunately, it is quite difficult to achieve direct liquid cooling in conventional iron lamination toothed stator designs. High copper fill factors and varnish-impregnated slot cells substantially inhibit the free flow of liquid coolant through the copper windings. In addition, since the copper is thermally isolated from the coolant by electrical insulators required between the copper windings and the laminations forming the stator core, cooling of the stator core is not substantially effective.
It is therefore apparent that a new design of the machine stator must include provision for effective liquid cooling if the high energy product permanent magnets are to be used effectively. It is therefore an object of the present invention to provide a stator for use with such magnets which may be effectively cooled by liquid coolant flow.
An additional problem encountered in the use of high energy product permanent magnets is that of providing a path for magnetic flux to flow through the stator core. Since a laminated stator core has a maximum flux density which may be effectively circulated therethrough, with the use of high energy product permanent magnets the size of the T-shaped radially inwardly extending teeth of the stator core must be increased. This increase in size in the teeth of the stator core results in less slot area for stator windings to occupy. Accordingly, while flux is increasing, slot area in which stator windings may be wound is simultaneously decreasing, thereby resulting in a performance plateau and in the ineffective use of the high energy product permanent magnets.
Two other problems associated with conventional stators are cogging and a relatively high magnetic spring rate. Cogging is a phenomenon whereby the preferred path of magnetic flux is the path of least reluctance. A rotor will have preferred positions it will move to at rest, resulting in a high cogging torque, which is the torque needed to overcome the cogging phenomenon. The solution to cogging has been a skewing of the stator teeth, resulting in a helical shape which produces a smooth running machine. It is desireable to eliminate cogging without skewing stator teeth.
Another problem is associated with the magnetic spring rate of a rotor within a stator. The magnetic spring rate is the force urging the rotor in a radial direction. It may be appreciated that such a force is applied to the bearings of the rotor, and will result in bearing wear. The only way to reduce the magnetic spring rate is to increase the magnetic air gap, and this is also an object of the present invention.
It is therefore necessary for an improved stator for permanent magnet machines to provide the maximum possible slot area for stator windings, and to prevent the slot area for stator windings from being diminished by the use of a higher energy product permanent magnet in the rotor. In fact, it is an object of the present invention to maximize the slot area by redesigning the construction of the stator to allow the maximum possible amount of stator windings to be installed on the stator.